It is well known in the art to envelop solid pharmaceutical materials, tablets, granules and seeds, for example, in a film covering as protection against oxidation, moisture, light, abrasion, rough handling, etc. A pigment is usually present as added protection against light and as a color aid for product identification. The film should be free of roughness, irregularities, cracks or mottled colorations. Film smoothness is important as an aid in swallowing. A hard, shiny surface is desirable for an attractive appearance. Of course, films and coating compositions for ingestion must be edible or physiologically compatible.
Film-forming compositions for coating pharmaceutical tablets preferably contain as the film-forming element, a film-forming resinous material, either naturally occurring or synthetic. Normally, film-forming compositions are applied as a liquid coating formulation comprising a liquid carrier medium having dispersed or dissolved therein the film-forming components. The liquid medium can be an organic solvent or water or a combination of both. Water is preferred owing to the risk of fire and toxicity from organic solvents. Also having to comply with governmental safety standards pertaining to the transportation and handling of industrial chemicals is another minus factor against solvent use.
Generally speaking, application of the liquid coating formulation is effected by spraying dry pharmaceutical forms in rotation in a coating pan or in a fluidized air bed. After evaporation of the liquid medium, the film coated pharmaceuticals are recovered.
As a commercial product, liquid coating compositions are unsatisfactory because of the high transportation costs due to the weight of the liquid carrier. Clearly, it is more practical and economical to ship coating compositions in dry form which can be reconstituted with the appropriate solvent or liquid by the pharmaceutical customer.
An example of a dry film-forming pharmaceutical coating composition and one which is made commercially is described in U.S. Pat. Nos. 4,543,370 and 4,683,256 to Porter et al. and assigned to Colorcon, Inc. In preparing such coating composition, the initial step consists in the high intensity blending of polymer and pigment particles in the presence of a plasticizer and optionally a surfactant. The resulting powder is then fine ground, after which it can be mixed with a solvent or water to provide a coating dispersion for application to tablets or other pharmaceutical forms. On drying, a uniform film is said to be produced on the coated substrate.
Film-forming polymers used in formulating the coating compositions of the patents include both water-soluble and water-insoluble types. Coating dispersions produced from the water-soluble polymers comprise an aqueous solution of the polymer in which are suspended the pigment particles. Water-soluble, film-forming polymers are advantageous in that coating dispersions can be prepared therefrom without the need for organic solvents and their attendant hazards.
The list of water-soluble, film-forming polymers enumerated in the cited patents are: hydroxypropylmethylcellulose, hydroxypropylcellulose, polyvinylpyrrolidone, methylcellulose and sodium ethylcellulose sulfate.
Although an advance in the art, the pharmaceutical coating system of the Colorcon patents is not entirely satisfactory for providing film coatings from solvent free aqueous dispersions owing to certain deleterious characteristics of the water-soluble polymers. For instance, hydroxypropyl-cellulose and polyvinylpyrrolidone produce sticky or tacky films on tablets when coated with aqueous dispersions of these polymers. Methocel is sometimes used but the film coatings tend to be dull and opaque. As far as sodium ethylcellulose sulfate is concerned, it is not normally used as a coating material.
Hydroxypropylmethylcellulose (HPMC), on the other hand, does give smooth transparent tack-free films from aqueous coating dispersion. It is the preferred polymer for coating pharmaceutical forms with the water-dispersible powders of the Colorcon patents. In fact, Colorcon manufactures and sells such dry HPMC film coating compositions under the trademark Opadry. However, coatings produced from HPMC-based Opadry powders are not as stable to moisture and temperature as might be desired. As a consequence, Opadry coated pharmaceuticals tend to deteriorate if exposed much above ambient humidity and temperature.